Radiometric imaging in the context of the present invention relates primarily (but not exclusively) to the field of millimeter wave imaging. Millimeter waves are radio waves in the wavelength range from approximately 1 mm to 10 mm, which corresponds to a frequency range from approximately 30 GHz to 300 GHz. These waves have been applied to in-vehicle radar devices for measuring the distance between moving vehicles in order to prevent collisions and to high speed wireless communications for achieving transmission data rate in the order of several gigabits per second. Further, improved generation and detection techniques as well as latest improvements in the integration and miniaturization of devices operating in the millimeter wave frequency range have created a lot of interest to exploit the properties of this electromagnetic radiation. Its ability to penetrate non-conducting materials, including plastics, walls, clothes, smoke and fog have provided additional momentum to research on millimeter waves imaging applications. Furthermore, the achievable spatial resolution of few millimeters is very interesting in security applications, such as airport security scanners for the detection of concealed weapons or explosives.
The millimeter waves can be used in an active or a passive way. A passive radiometric imaging system creates images by capturing the electromagnetic radiation emitted by the objects by using a radiometer. A radiometric body scanner, for example, detects the millimeter waves which are naturally emitted by the human body in order to reconstruct an image of the body contours. A radiometric body scanner is, however, not limited to reconstructing an image of the contours of the body itself, but may also image the entire surface of the body like in an ordinary picture. Such a passive radiometric imaging system is, for instance, known from US 2007/0221847 A1. An active radiometric imaging system, on the other hand, illuminates objects with millimeter waves and creates images by detecting the transmitted or reflected waves. Millimeter waves do not produce ionizing effects, which makes this technology an attractive candidate to be employed in security applications like concealed weapon or explosives detection, while existing technologies based on infrared or visible radiation cannot detect concealed objects, and X-ray based systems cannot be used due to its ionizing effect.